<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style type="text/css">
.wrap {
	background: yellow;
	width: 200px;
	height: 200px;
	position: relative;
}

.center {
	background: green;
	position: absolute;
	width: 100px;
	height: 100px;
	left: 50%;
	top: 50%;
	margin-left: -50px;
	margin-top: -50px;
}
</style>
</head>

<body>
	<div class="wrap">
		<div class="center"></div>
	</div>
</body>
<script>
alert(a); // undefined
alert(b); // "b" 没有声明
 
b = 10;
var a = 20;
</script>
</html>
